1. Field of the Invention
The present invention relates to an abnormality determining apparatus for an engine. More particularly, this invention relates to an apparatus which determines the existence of an abnormality in the fuel supply system of an engine when a parameter value used for feedback control of the fuel air ratio of the engine continuously satisfies a predetermined condition for a predetermined period of time.
2. Description of the Related Art
Japanese Unexamined Patent Publication No. 63-1753 discloses a conventional abnormality determining apparatus for a fuel air ratio control system for a motor vehicle engine. The disclosed engine system is provided with an apparatus for absorbing fuel vapor in a fuel tank, and a fuel vapor purge passage for connecting the absorbing apparatus to an air-intake passage. A valve for controlling the purging of fuel is disposed in the purge passage. The fuel vapor absorbing apparatus inhibits fuel vapor in the fuel tank from being discharged in the air. The purge passage supplies the fuel, collected by the absorbing apparatus, to the air-intake passage. Based on the output signal of an exhaust gas sensor disposed in an exhaust passage of the engine, an engine control unit performs feedback control of the fuel air ratio.
According to the conventional abnormality determining apparatus, when the feedback value of the fuel air ratio continuously exceeds a predetermined limit for a first predetermined period of time, the purge control valve is temporarily closed to block the purge passage. In addition, the control unit starts measuring the time since the closing of the purge control valve. When the feedback value continuously exceeds the predetermined limit value for a second predetermined period of time, even with the purge passage blocked, the control unit determines that an abnormality has occurred in the fuel air ratio control system. This method of diagnosing the fuel air ratio eliminates the need to consider the effects of purging the fuel vapor from the fuel delivery system. Even if the feedback value of the fuel air ratio exceeds the predetermined limit value due to the influence of the fuel vapor purging, engine abnormalities can be detected without error. According to the conventional art, however, the purge control valve is temporarily closed during the diagnosis of the fuel air ratio control system. This effectively prevents fuel vapor purging from being executed during the diagnosis. When the purge control value closes, vapor pressure continually builds up in the fuel tank causing an unavoidable discharge of fuel vapor from the fuel tank to the atmosphere.
To overcome this shortcoming, the process of determining whether abnormalities exist in the fuel air ratio control system may be suspended during fuel vapor purging. This would eliminate the need to consider the influence which the fuel vapor purge has on the diagnosis of the fuel system. This would also prevent fuel vapor from being discharged into the air during fuel vapor purging. According to this method, however, it is impossible to properly diagnose an abnormality in the fuel air ratio control system during the fuel vapor purging. This in turn substantially reduces the accuracy or reliability of diagnosing engine condition abnormalities.